1. Field of the Invention
This invention relates to a dielectric ceramic composition having a high dielectric constant and a high Q value in a high frequency region such as a microwave and a millimeter wave and a preferred temperature coefficient of a resonance frequency.
2. Description of the Prior Art
In a high frequency region such as a microwave and a millimeter wave, dielectric ceramics are widely used as a dielectric resonator or a dielectric substrate for MIC.
For example, ZrO.sub.2 -SnO.sub.2 -TiO.sub.2 type materials, BaO-TiO.sub.2 materials, (Ba, Sr) (Zr, Ti)O.sub.3 type materials, and Ba (Zn, Ta)O.sub.3 type materials have been known as dielectric ceramics of this type. These materials have a dielectric constant of 20 to 30 and a Q value of 1000 to 3000 at a frequency of 500 MHz to 5 GHz by various improvement and further possess a temperature coefficient (.tau.f) of a resonance frequency of about 0 ppm/.degree.C.
However, the frequencies used recently have tended to become higher and dielectric materials have been required to have superior dielectric properties, especially a high Q value. But conventional dielectric materials do not at present have a high Q value of a high practical level in a used frequency region of 10 GHz.
The present inventors previously proposed a dielectric ceramic composition comprising a complex oxide containing BaO, MgO and WO.sub.3 as a composition having a high dielectric constant and a high Q value in a high frequency region which can meet this requirement (Japanese Patent Application No. 284,470/1991).
However, a high Q value can be obtained in this dielectric ceramic composition, but since the temperature coefficient (.tau.f) of the resonance frequency is too much biased to a negative side, its field of utilization is limited when it is used as a microwave dielectric ceramic material. Thus, there is a problem in the field of practical utilization.
The present inventors have investigated the above problem, and as a result, have found that the temperature coefficient (.tau.f) of the resonance frequency can be transferred to a suitable value on a more positive side by including a predetermined amount of at lease one element selected from Group 3b elements, Group 4b elements in the periodic table, Sn, Mn and Ca in a complex oxide expressed by BaO-MgO-WO.sub.3. Thus, the present invention has been completed.